1. Field of the Invention
The present invention relates to an ultrasonic diagnostic apparatus and a control method thereof, and more particularly, to an ultrasonic diagnostic apparatus configured to three-dimensionally scan the inside of a body under examination using an ultrasonic wave in response to a trigger signal generated based on an electrocardgram signal or the like, and a method of controlling such an ultrasonic diagnostic apparatus.
2. Description of the Related Art
In recent years, an ultrasonic diagnostic apparatus capable of displaying a three-dimensional moving image has been in active development, and it has become possible to display a three-dimensional diagnostic image with higher resolution over a larger region than in conventional two-dimensional images.
The ultrasonic diagnostic apparatus generates a diagnosis image using an ultrasonic wave propagating in a living body, and thus the time from the transmission of an ultrasonic pulse to the reception of a reflected wave from a living body is basically the same for a three-dimensional ultrasonic diagnostic apparatus and a two-dimensional ultrasonic diagnostic apparatus. To scan a three-dimensional region in a living body with high resolution, a great number of scanning beams are required. Thus, the three-dimensional ultrasonic diagnostic apparatus generally needs a longer time to scan a specified region than the two-dimensional ultrasonic diagnostic apparatus needs. In other words, when the spatial resolution is equal, the frame rate of the three-dimensional image (i.e., the frequency at which the three-dimensional image is updated) obtained by the three-dimensional ultrasonic diagnostic apparatus is theoretically lower than the frame rate of the two-dimensional image obtained by the two-dimensional ultrasonic diagnostic apparatus.
To solve the problem described above, various techniques have been proposed (see, for example, U.S. Pat. No. 6,544,175, JP-A 2007-20908, JP 3,046,424 B etc.). A basic idea of these techniques is to divide a full region (volume) under examination for diagnosis (hereinafter, referred to simply as a full volume) into a plurality of small regions (hereinafter referred to as sub volumes), and obtain a three-dimensional image of the full volume by connecting image data obtained by scanning three-dimensional space of the sub volumes at a high frame rate. In this technique, the observation time of sub volumes is different from each other. Therefore, it is important to connect sub volumes so that good spatial continuity is achieved.
Depending on a part under diagnosis, the part can move due to breathing or a heartbeat. To avoid a problem due to the motion of the part under diagnosis, for example, U.S. Pat. No. 6,544,175 discloses a technique to acquire a plurality of image data in a sub volume in synchronization with the motion of a heart. In this technique disclosed in U.S. Pat. No. 6,544,175, a three-dimensional moving image of a heart is produced in real time as described briefly below.
In this technique, a signal of an electrocardiogram, i.e., an ECG signal is used as a signal synchronous with motion of a heart. More specifically, an R-wave signal, which appears at the end of a diastolic period, is used as an ECG trigger signal.
A three-dimensional full volume of a heart under examination is divided into four sub volumes, and image data of one heartbeat is captured in synchronization with the ECG trigger signal for each sub volume. Note that the image data of one heartbeat includes a plurality of frames of images. For example, 20 frames of images of one sub volume are obtained by repeatedly scanning the sub volume 20 times for one heartbeat (during one interval of the ECG trigger signal). In this case, if the repetition period of the heartbeat is assumed to be one second, the image data of each sub volume is obtained at a frame rate of 20 fps, which is reasonably high to obtain a moving image representing motion of a heart.
The plurality of frames of image data obtained for each sub volume are connected to obtain a full volume of image data as follows. That is, frame images that are same in “time phase” are extracted from the plurality of fame images of sub volumes and are connected together so as to obtain a frame image of the full volume. The “time phase” refers to a delay with respect to a time at which an ECG trigger signal is generated. The motion associated with contraction and relaxation of the heart normally has periodicity synchronous with the ECG trigger signal. Therefore, by extracting frame images which are equal in the time phase from the respective sub volumes and connecting the extracted frame images, it is possible to obtain good spatial continuity between the sub volumes. In practice, successive “time phase numbers” are assigned to frame images in scanning order from one closest to an ECG trigger signal, and an image of a full volume is synthesized by connecting frame images having an equal time phase number. For example, in a case where the full volume is divided into four sub volumes A, B, C, and D and each sub volume is scanned repeatedly 20 times, a total of twenty frame images with time phase numbers of 0 to 19 are obtained for each sub volume. Frame images with each equal time phase number are extracted from the sub volumes A, B, C, and D and the extracted frame images are connected together thereby obtaining a synthesized image of the full volume corresponding to the time phase number. The combining of frame images is performed for each of the time phase numbers so as to obtain synthesized full volume images with time phase numbers from 0 to 19. Thus, a total of twenty synthesized full volume frame images are obtained for each ECG trigger signal. Note that the frame rate of the full volume images is equal to that of the sub volume images. Thus, for example, a full volume moving image with a frame rate of 20 fps is obtained.
As described above, in the conventional techniques, each sub volume is scanned a plurality of times in response to each ECG trigger signal so that each scanning provides one frame image (a frame image of a sub volume). The number of repetitions of scanning for each sub volume is predetermined based on the period of the ECG trigger signal before diagnosis using a three-dimensional image is started.
However, our heartbeat period is not necessarily constant. On the contrary, even persons with a normal and healthy body have a variation of about 10% in the heartbeat period. In the case of patients having a disease such as arrhythmias, a greater variation in heartbeat period can occur. Thus, the period of the ECG trigger signal also varies according to the variation in heartbeat period.
Therefore, the number of repetitions of scanning for each sub volume is not necessarily equal to a value determined before diagnosis is started. For example, even if the number, N, of repetitions of scanning for each sub volume is set to 20 before diagnosis is started, the number, N, of repetition of scanning can vary to a lower value such as 18 or a higher value such as 22 depending on a variation in heartbeat period occurring after the diagnosis is started. This can cause such a problem that when frame images with the same phase number are tried to be connected, there is no scanning data for some sub volume.
For example, if the heartbeat period is short in a scan period for a sub volume A and thus scanning data is obtained for only time phase numbers from 1 to 18, and if the heartbeat period becomes longer for sub volumes B, C, and D following the sub volume A and thus scanning data is obtained for all time phase numbers from 1 to 20, then the following problem can occur. When it is attempted to produce a full volume image by connecting frame images with a time phase number 19 or 20, it is impossible to achieve spatial continuity in the full volume because there is no frame image with a time phase number 19 or 20 for the sub volume A. Furthermore, for example, in the sub volume A, the maximum obtainable time phase number varies depending on the fluctuation of the heartbeat period, and thus scanning data can be acquired only up to the time phase number 18 in a particular period, while scanning data can be acquired up to the time phase number 20 in another period. As a result, when the full volume image is displayed in the form of a moving image, temporal continuity is not obtained in the sub volume A.
As described above, in the conventional techniques, there is a possibility that spatial or temporal discontinuity occurs in the image due to fluctuations of the heartbeat period. The discontinuity of the image can cause a problem in diagnosis using the image.
In view of the above, it is an object of the present invention to provide an ultrasonic diagnostic apparatus capable of preventing or reducing spatial/temporal discontinuity in a synthesized image regardless of a fluctuation of the heartbeat period, and a method of controlling such an ultrasonic diagnostic apparatus.